1. Field of the Invention
This invention relates to optical fibers. More specifically, this invention relates to optical fibers intended for use in a device which makes use of the evanescent wave.
While the present invention is described herein with reference to a particular embodiment, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof.
2. Description of the Related Art
In conventional fiber optic sensors light guided within an optical fiber is modified in reaction to various external physical, chemical or similar influences. Light from a source is coupled into the fiber and subsequently propagates to a region in which a measurement is to take place. In extrinsic sensors the guided light then may exit the fiber and interact with the substance being measured (measurand) prior to being relaunched into the same or a different fiber. Alternatively, in intrinsic sensors the light remains within the fiber throughout the measurement region.
In another type of optical fiber sensor, generally known as an evanescent wave sensor, the light guided by the fiber is partially conducted via the evanescent (i.e. exponentially decaying) wave which surrounds the core. In evanescent wave sensors a cladding sheath surrounds the optical fiber core creating a cladding-core interface such that an evanescent wave is supported by the cladding. The evanescent wave comprises a portion of the optical energy injected into the fiber which propagates along the length of the fiber within the cladding. The measurand surrounding the fiber may either absorb or change the properties of the evanescent wave, thus enabling a measurement to be performed. Employment of evanescent wave coupling into the measurand may be preferred over extrinsic mode fiber sensors in applications requiring direct interaction between the light beam and the measurand since the former approach requires no relaunching of the beam.
Unfortunately, the extremely small cross-sectional areas of fibers used in evanescent measuring devices generally render the fibers fragile and difficult to manipulate. While conventional optical fibers may have a standard cross-sectional diameter on the order of 120 .mu.m, evanescent wave fibers often have a cross-sectional diameter of less than 15 .mu.m. This small diameter arises as a consequence of the requirement that the cladding layer surrounding the core be sufficiently small to permit measurable coupling into the measurand.
Accordingly, a need in the art exists for an optical fiber which can be used to take advantage of the evanescent wave yet still have sufficient cross-sectional area to afford ease of manipulation.